Carbide nanostructures and methods for making same

ABSTRACT

A structure includes a substrate and a metallized carbon nano-structure extending from a portion of the substrate. In a method of making a metallized carbon nanostructure, at least one carbon structure formed on a substrate is placed in a furnace. A metallic vapor is applied to the carbon nanostructure at a preselected temperature for a preselected period of time so that a metallized nanostructure forms about the carbon nanostructure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Division of U.S. patent application Ser. No.10/986,599, filed Nov. 10, 2004, entitled CARBIDE NANOSTRUCTURES ANDMETHODS FOR MAKING SAME, which is herein incorporated by reference.

FEDERALLY SPONSORED RESEARCH

The United States Government may have certain rights in this inventionpursuant to Cooperative Agreement No. 70NANB2H3030, awarded by theNational Institute of Standards and Technology, United States Departmentof Commerce.

BACKGROUND

1. Field of the Invention

The invention relates to nano-scale structures and, more specifically,to a method of making nano-structures by carbon nanotube confinedreactions.

2. Description of the Prior Art

Field emission devices (gated or ungated) have applications in X-rayimaging systems, displays, electronics, microwave amplifiers,fluorescent lamp cathodes, gas discharge tubes, and many otherelectrical systems. Other applications for field emission devicesinclude sensors, photonic bandgap devices, and wide bandgapsemiconductor devices.

Carbon nanotubes are currently being researched as electron emissionsources in, for example, flat panel field emission display applications,microwave power amplifier applications, transistor applications andelectron-beam lithography applications. The carbon nanotubes aretypically synthesized through an arc discharge method, a chemical vapordeposition (CVD) method or a laser ablation method. Carbon nanotubesoffer the advantage of having high aspect ratios which increases thefield enhancement factor and therefore the extraction of electrons atrelatively low electric fields. Carbon nanotubes, however, exhibit afairly high work function, and are prone to damage under typicaloperating conditions, limiting the life and effectiveness of thedevices. What is needed, therefore, is a material more robust and with alower work function than carbon, but with a cylindrical geometry anddiameters in the 10-800 nm range.

Metal carbide nanorods are candidate materials for use in field emissiongated devices, which have applications in stationary computed tomographysystems, displays, etc. A fabrication procedure is required that allowsfor seamless integration with gated device structures as well as controlof the lateral density of nanorods so that electric field shielding doesnot occur. Carbide materials may be preferred due to their chemicalstability, mechanical hardness and strength, high electricalconductivity, and relatively low work function. These characteristicsmake them particularly suited to the environment that may be found in acomputed tomography system and the like. Such materials may also beimportant in superconducting nanodevices, optoelectronic nanodevices,display systems, lighting systems and other similar systems.

The main approach to synthesizing carbide nanorods to date has been touse a carbon nanotube (CNT) as a template on which a reaction is carriedout between the CNT and a metal, metal oxide, or metal iodide in vaporform to produce metal carbide nanorods. This process is typically donein a vacuum-sealed tube with reaction times being more than 24 hours.However, demonstration of CNT conversion on a substrate such as siliconor in a device structure is not known.

Therefore, there is a need for a system that generates metallized carbonnanostructures from a substrate in a relatively short period of time.

There is also a need for a system that grows elongated nanostructures insitu directly in device structures.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the presentinvention, which, in one aspect, is a structure that includes asubstrate and a metallized carbon nano-structure extending from aportion of the substrate.

In another aspect, the invention is a field emitter that includes aconductive substrate, an insulating layer, a conductive gate layer, andat least one metallized carbon nano-structure. The conductive substratehas a top surface. The insulating layer is disposed adjacent to the topsurface of the conductive layer and defines a passage that exposes aportion of the top surface of the substrate. The conductive gate layeris disposed adjacent to the insulating layer and defines a hole that isin substantial alignment with the passage defined by the insulatinglayer. The metallized carbon nano-structure extends from the portion ofthe top layer of the substrate through at least a portion of the passagedefined by the insulating layer.

In another aspect, the invention is a method of making a metallizedcarbon nanostructure, in which at least one carbon structure formed on asubstrate is placed in an furnace. A metallic vapor is applied to thecarbon nanostructure at a preselected temperature for a preselectedperiod of time so that a metallized nanostructure forms about the carbonnanostructure.

In yet another aspect, the invention is a method of making a fieldemitter, in which an insulating layer is placed on a conductivesubstrate. A conductive gate layer is placed on the insulating layer,opposite the conductive substrate. An opening is defined in theconductive gate layer and the insulating layer, so as to expose aportion of the conductive substrate. At least one carbon nanostructureis grown on the exposed portion of the substrate. A metal vapor isapplied to the carbon nanotube so as to form a metallized carbonnanostructure in the opening.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 is a schematic view of a tube furnace used in a method accordingto one illustrative embodiment of the invention.

FIG. 2A is a cross-sectional view showing a plurality of carbonnanotubes extending from a substrate.

FIG. 2B is a cross-sectional view showing a plurality of metallizedcarbon nanotubes extending from a substrate.

FIG. 3A a perspective view of a field emitter.

FIG. 3B is a cross-sectional view of the field emitter shown in FIG. 3A,taken along line 3B-3B.

FIG. 3C is a cross-sectional view of a field emitter in which aconductive layer is deposited on the substrate.

FIG. 3D is a cross-sectional view of a field emitter in which thesubstrate includes a raised portion.

FIG. 4A is a micrograph of a carbon nanotube metallized with an oxide.

FIG. 4B is a micrograph of a carbon nanotube metallized with a metal.

FIG. 4C is a micrograph of a cross section of NbC nano-structures on asubstrate.

FIG. 5A is a cross-sectional view of a field emitter diodeconfiguration.

FIG. 5B is a cross-sectional view of a field emitter triodeconfiguration.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. As used in the description herein and throughout the claims,the following terms take the meanings explicitly associated herein,unless the context clearly dictates otherwise: the meaning of “a,” “an,”and “the” includes plural reference, the meaning of “in” includes “in”and “on.” Unless otherwise specified herein, the drawings are notnecessarily drawn to scale.

Also, as used herein, nanostructure means a structure having a narrowestdimension diameter of less than 800 nanometers (nm). as used herein,“metalized carbon nanostructures” include metal carbide nanostructuresand metal carbide structures with carbon nanotube cores.

Metal carbide nanostructures are made as disclosed below. Suchnanostructures may be used to form field emitting devices. Initiallycarbon nanotubes, either grown from a substrate through methods known tothe art (such as by an in situ catalyst-based carbon nanotube procedure)or deposited onto a substrate in the form of a paste, are exposed to ametal vapor and form metallized nanostructures. Different types ofcarbon nanotubes (e.g., single-walled nanotubes, double-walled nanotubesand multi-walled nanotubes) may be employed. The type of nanotubeemployed will affect the properties of the resulting nanostructures.

In one example of a method of making metallized carbon nanostructures,as shown in FIG. 1, carbon nanotubes 112, or other carbonnanostructures, disposed on a substrate 110 are placed in an furnace 100(such as a quartz tube furnace), along with a metal precursor 120. Themetal precursor 120 could be a metal, such as a transition metal, or anoxide of a metal. Examples of suitable metals include: molybdenum,tungsten, zirconium, hafnium, silicon, niobium, titanium, and tantalum.Combinations of these metals, or oxides of these metals, or iodides,bromides, organometallics, etc. employing these metals may also givedesirable results. The use of a metal oxide as the metal precursor 120may be desirable in some embodiments because the resulting vaporpressure may be higher than with a non-oxidized metal.

The furnace 100 is heated to a preselected temperature for a preselectedperiod of time so that a least a portion of the metal precursor 120vaporizes and metallized nanostructures form about the carbonnanostructures 112 as part of a chemical vapor deposition process. A gasis used to carry the metal vapor to the carbon nanotubes 112.

In one illustrative example, an inert gas (e.g., argon) at a gas flowrate of between 300 sccm and 1000 sccm, is forced to flow through thefurnace at a predetermined gas flow rate, entering at a gas intake 102and flowing out through an exit 104. Alternatively, a combination of aninert gas and a reducing gas may be used. For example, the inert gascould include argon applied at a gas flow rate of between 300 sccm and1000 sccm, and the reducing gas could include hydrogen at a gas flowrate that comprises between 1 sccm and 300 sccm. Adding a reducing gasreduces the effects of residual oxygen in the furnace 100 associatedwith some precursors. In one exemplary embodiment of the method, thepressure inside the furnace 100 is approximately one atmosphere, thetemperature is maintained in a range from about 800 K to about 1200 Kand the metallized carbon nanostructures are formed in about one to fourhours. In this embodiment, using molybdenum oxide as the metalprecursor, the following reaction takes place:

As shown in FIG. 2A, in executing the above-described method, aplurality of carbon nanotubes 112, which extend from the substrate 110(which may include a material such as, for example, silicon or siliconcarbide), are placed in the furnace. As shown in FIG. 2B, the chemicalvapor deposition process transforms the carbon nanotubes into metallizedcarbon nanostructures 114. Depending on the time and temperatureparameters employed, and the type of carbon nanotube used, themetallized carbon nanostructures 114 could be metal carbide throughout,they could have a carbon core with a carbide exterior, or they couldhave a carbon or carbide core surrounded by a metal sheath.

Selection of the metal influences the work function of the resultingdevice. The following table lists work functions associated with variousmetal carbides, and gallium nitride (GaN), which is included forcomparison:

Material Work Function (eV) Mo₂C  3.8-4.74 WC 3.6  ZrC  2.1-4.39 HfC2.04-4.15 SiC (0001) 4.58 NbC 2.24-4.1  TiC 2.35-4.12 GaN 2.9-3.9

As shown in FIGS. 3A and 3B, a field emitter 300 may be constructedaccording to the above-disclosed method. The field emitter 300 includesa conductive substrate 310 having a top surface 312. An insulating layer320 is disposed adjacent to the top surface 312 of the conductive layer310. The insulating layer 320 defines a passage 316 that exposes aportion of the top surface 312 of the substrate 310. A conductive gatelayer 330 is disposed adjacent to the insulating layer 320 and defines ahole 332 that is in substantial alignment with the passage 316 definedby the insulating layer. A plurality of metallized carbonnano-structures 314 extends from the portion of the top layer 312 of thesubstrate 310 through at least a portion of the passage 316 defined bythe insulating layer 320.

As shown in FIG. 3C, the conductive substrate 310 could include aconductive layer 311 and a conductive film 318 (which could includetitanium nitride) that acts as a diffusion barrier disposed on theconductive layer 311.

As shown in FIG. 3D, the substrate 310 may include a raised portion 322extending from a top surface 312 of the substrate 310. The raisedportion 322 could be conical in shape. The metallized carbonnano-structures 314 extend upwardly from the raised portion 322.

Two micrographs of structures made according to the disclosed method areshown in FIGS. 4A and 4B. These figures show carbide structures in whichthe following reaction occurred:2NbI₅+2C→2NbC+5I₂.FIG. 4A shows several metallized carbon nano-structures formed frommetallization of carbon nanotubes 412 in which the metal precursor usedwas a metal iodide. A cross-sectional micrograph of NbC nano-structures420 on a substrate 422, made according to the method disclosed herein,is shown in FIG. 4C.

As shown in FIGS. 5A and 5B, the addition of an anode 502 may be used inconstructing an active electronic device. For example, FIG. 5A shows adiode 500 configuration of a field emitter and FIG. 5B shows a triode510 configuration of a field emitter. If the anode 502 is coated with aphosphor, then the active device may generate light when a suitablepotential is applied. Such devices could be used in imaging systems,such as, for example, x-ray imaging systems, displays, and in lightingsystems. Devices made as disclosed herein would result in a lower workfunction than conventional field emitters and make systems employingfield emitters more robust. Also, as is understood, there are many fieldemitter configurations that could be made according with the presentdisclosure (e.g., different spatial dispositions of the gate, differentmaterials added to the anode, etc.).

Field emitter 300 may also be serve as a cathode to generate an electronbeam in an x-ray generating device, or x-ray source. In thisapplication, a plurality of field emitters 300 are arranged in an array.Electrons emitted by field emitters 300 impinge upon an anode, which inturn emits x-rays. Such an x-ray source may be used in imaging systems,such as computer tomography systems and the like, in medical,inspection, and other radiographic applications. The use of fieldemitters as x-ray sources are described in U.S. Pat. No. 6,385,292 byBruce M. Dunham et al., entitled “Solid-State CT System and Method,”issued on May 7, 2002; United States Patent Application Publication US2002/0085674 by Bruce M. Dunham et al., entitled “Radiography Devicewith Flat Panel X-Ray Device,” filed on Dec. 29, 2000; and United StatesPatent Application Publication US 2003/0002628 by Colin R. Wilson etal., entitled “Method and System for Generating an Electron Beam inX-Ray Generating Devices,” filed on Jun. 27, 2001, the contents of whichare incorporated by reference herein in their entirety.

Additional coatings may be applied to the resulting devices to achievedesired properties. For example, certain coatings to the structures(which could be applied in a chemical vapor process or through someother chemical process) could lower the work function of the devicesand/or could provide additional protection from ion bombardment. Anexample of this is coating of the carbide nanostructures by a boronnitride or barium oxide coating.

One typical method of growing carbon nanotubes includes placing a filmof a catalyst on a substrate, heating the catalyst so that thin filmforms islands, and applying a carbon compound to the islands to growcarbon nanotubes. The catalyst islands may remain at substrate or theymay stay at the growing end of the carbon nanotube (or they may even beincorporated within the nanotube).

The above described embodiments are given as illustrative examples only.It will be readily appreciated that many deviations may be made from thespecific embodiments disclosed in this specification without departingfrom the invention. Accordingly, the scope of the invention is to bedetermined by the claims below rather than being limited to thespecifically described embodiments above.

What is claimed is:
 1. A field emitter, comprising: a. a conductivesubstrate, having a top surface; b. an insulating layer, disposedadjacent to the top surface of the conductive substrate, the insulatinglayer defining a passage that exposes a portion of the top surface ofthe substrate; c. a conductive gate layer, disposed adjacent to theinsulating layer and defining a hole that is in substantial alignmentwith the passage defined by the insulating layer; and d. at least onemetallized carbon nano-structure extending from the portion of the toplayer of the substrate through at least a portion of the passage definedby the insulating layer, wherein the metallized carbon nano-structurecomprises a carbon core and a metal carbide exterior.
 2. The fieldemitter of claim 1, wherein the conductive substrate comprises silicon.3. The field emitter of claim 1, wherein the conductive substratecomprises silicon carbide.
 4. The field emitter of claim 1, wherein theconductive substrate comprises: a. a non-conductive layer; and b. aconductive film disposed on the non-conductive layer.
 5. The fieldemitter of claim 4, wherein the non-conductive layer comprises silicondioxide.
 6. The field emitter of claim 5, wherein the field emitter isemployed in an imaging system.
 7. The field emitter of claim 5, whereinthe field emitter is employed in a lighting system.
 8. The field emitterof claim 5, wherein the field emitter is employed in a display.
 9. Thefield emitter of claim 1, wherein the substrate includes a raisedportion extending from a top surface of the substrate, the metallizedcarbon nano-structure extending upwardly from the raised portion. 10.The field emitter of claim 9, wherein the raised portion is partiallyconical in shape.
 11. A structure, comprising: a substrate; and ametallized carbon nano-structure extending from a portion of thesubstrate, wherein the metallized carbon nano-structure comprises acarbon core and a metal carbide exterior.
 12. The structure of claim 11,wherein the metallized carbon nanostructure includes a metal selectedfrom a list consisting of: molybdenum, tungsten, zirconium, hafnium,silicon, niobium, titanium, tantalum, and combinations thereof.
 13. Thestructure of claim 11, wherein the metal carbide exterior is formed froma carbon nano-tube that has reacted with a metal.
 14. The structure ofclaim 11, wherein the metal carbide exterior is formed from a carbonnano-tube that has reacted with a metal oxide.
 15. The structure ofclaim 11, wherein the carbon core transitions into a metal carbideexterior.
 16. The structure of claim 11, wherein the structure forms aportion of an x-ray source.